Mid-Infrared Linear Dichroism Investigation of the Pyridine-Iodine System in Stretched Polyethylene
It is well known that pyridine (py) and iodine (I2) form a 1:1 molecular complex, py-I2. Within the framework of a student project at Roskilde University , we have recently investigated the formation and mid-infrared linear dichroism of the complex in a stretched polyethylene (PE) matrix. However, the observed spectra displayed not only the expected transitions due to py and py-I2, but also a third, prominent series of sharp transitions, evidently due to one or more additional products. On the basis of a comparison with literature spectra, we assigned these transitions to the N-iodopyridinium cation (pyI+). This assignment is consistent with the observed polarization data. Ionic side reactions (py-I2 = pyI+ + I–, py-I2 + I2 = pyI+ + I3–, etc.) are easily observed in binary pyridine-iodine mictures and in polar solvents, but not so easily in non-polar media. We are thus surprised to see the apparent efficiency with which the pyI+ cation is formed in an apolar and aprotic medium like PE. With excess iodine, the peaks assigned to pyI+ dominate the observed mid-infrared spectrum. We suspect that a driving force for the ionic reaction is the formation of polyiodide anions in the channels of the polymer medium. Further investigation of the phenomenon is in progress.  H. S. van Deurs, M. L. M. Espersen, R. Nissen, M. H. Rasmussen, C. J. S. Schönbeck, Student Project, Roskilde University, June 2006 (in Danish).
Xviii International Conference on Physical Organic Chemistry: Icpoc-18, August 20-25, 2006, 2006, p. 42-43
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International Conference on Physical Organic Chemistry (ICPOC), 2006